1. Field of the Invention
The present invention relates to an oscillation circuit for generating reference clock signals for use in operations of a semiconductor device.
2. Description of the Related Art
Since various functions can be realized by a plurality of integrated circuits, a semiconductor device is used as a microcomputer, microprocessor or the like in various apparatuses. Each circuit incorporated in the semiconductor device operates in response to a reference clock signal. The reference clock signal is generated based on an oscillation signal outputted from an oscillation circuit incorporated in the semiconductor device. Specifically, an output signal from the oscillation circuit is used as the reference clock signal as it is, or the output signal from the oscillation circuit is obtained via an inverter for use as the reference clock signal.
As one oscillation circuit there is a constitution using a comparator. The oscillation circuit constituted using the comparator is disclosed, for example, in a following document: Japanese Patent Application Laid-Open No. 65046/1996.
As disclosed in the document, a signal transmitted to a plus input terminal of a comparator and a signal transmitted to a minus input terminal are compared to output a comparison result as an oscillation signal. Moreover, the signals transmitted to two input terminals of the comparator are both dependent on a resistance element.
Environments where the semiconductor device is operated are various. Therefore, the semiconductor device may be operated under temperatures higher than an ordinary temperature or under temperatures lower than the ordinary temperature.
In the operation under such environment, a frequency of the oscillation signal outputted by the oscillation circuit fluctuates. Specifically, the signal transmitted to the input terminal of the comparator is dependent on the resistance element easily influenced by the temperature, and a resistance value of the resistance element fluctuates by the temperature.
For example, when the output signal of the comparator is fed back to one input terminal of the comparator via the resistance element, the feedback input of the output signal is retarded or accelerated by the fluctuation of the resistance value of the resistance element. On the other hand, a voltage of a reference signal transmitted as a comparison reference to the other input terminal of the comparator is subjected to potential dividing by a plurality of resistance elements. Since the voltage depends on a ratio of the resistance value of a desired resistance element relative to the total of resistance values of the plurality of resistance elements, it hardly fluctuates.
Since the comparator performs a comparison processing by these two input signals, a switch cycle of a voltage level of the output signal (oscillation signal) fluctuates. As a result, the frequency of the reference clock signal fluctuates. The fluctuation of the frequency of the reference clock signal inhibits each circuit from normally operating based on the reference clock signal inside the semiconductor device.
An object of the present invention is to provide an oscillation circuit for following temperature changes to output an oscillation signal of a stable frequency under any temperature.
Moreover, an object of the present invention is to provide an oscillation circuit to realize the above-mentioned object, so that enlargement of a circuit scale and complication of manufacture are minimized.